Understanding the structure and properties of geological formations can reduce the cost of drilling wells for oil and gas exploration. Measurements made in a borehole (i.e., down hole measurements) are typically performed to attain this understanding, to identify the composition and distribution of material that surrounds the measurement device down hole. To obtain such measurements, logging tools of the acoustic type are often used to provide information that is directly related to geo-mechanical properties.
Traditional acoustic tools utilize transmitters to create pressure waves inside the borehole fluid, which in turn create several types of waveguide modes in the borehole. Corresponding modes of propagation occur in the formation surrounding the borehole, and each of these can be used to provide information about formation properties. Thus, data associated with the various modes can be acquired and processed to determine formation properties, such as compression and shear wave velocity in the formation. For this reason, acoustic tools are an integral part of modern geophysical surveys, providing information on the mechanical properties of the medium by measuring acoustic modes of propagation.
When using conventional acoustic tools, at least two difficulties arise. First, the velocity of sound in the fluid surrounding the tool may vary, causing errors in a variety of distance (e.g., caliper) measurements. Second, reflector signals provided by the interface behind the casing are often troublesome to separate from other signals, so that the location of the interface itself is poorly defined.